1. Technical Field
The present invention relates to inkjet printers. More particularly, the present invention relates to an improved masking technique for reducing or eliminating banding artifacts and ink migration in output produced by inkjet printers.
2. Description of the Prior Art
State of the art inkjet printers form an image on a recording medium by depositing a pattern of ink drops onto the medium surface. FIG. 1 is a perspective view of a typical inkjet printer 10 which includes a plurality of print heads that are used alone and in combination to print color images on the recording medium 16. A complement of print heads includes a cyan print head ("C") 12, a yellow print head ("Y") 13, a magenta print head ("M") 14, and a black print head ("K") 15. By applying ink from these print heads in a specified pattern, the printer forms high resolution color images on the surface of the recording medium.
FIG. 2 is a perspective view of a typical print head of the type used with the printer shown in FIG. 1. The print head 20 is removably fitted into a recess (not shown) formed in the printer carriage. A contact array 22 makes an electrical connection between the print head and the printer to couple various control signals to the print head that determine when and where the print head is to discharge ink. An array of ink ports 24 (also referred to as ink discharge nozzles) is oriented such that ink is discharged substantially perpendicular to the plane of the recording medium 25.
The print head is operated to form a selected pattern on the nozzle array in accordance with the control signals applied thereto. Application of such control signals opens and muffles the various ink ports to produce a selected ink discharge pattern at each print location on the recording medium surface. The print locations are referred to as pixels; the process of producing the ink discharge pattern is referred to as masking.
In known printers, the print head is traversed back and forth across the surface of the recording medium to print a swath. The printing medium is incrementally advanced periodically to permit the array of print head nozzles to pass over another swath.
The image formed by the printer is built up incrementally on the surface of the recording medium from thousands of dots. The quality of the image is ultimately dependent upon the quality of the individual dots and their interrelationship to each other. Ideally, each dot is a generally circular, well formed spot of dried ink that fills its proper pixel to the pixel boundaries without extending into neighboring pixels.
A number of factors can prevent the formation of such an ideal array of dots. For example, horizontal bands can be seen consistently in the dark areas of images printed by inkjet printers. Such printers provide a color enhanced mode, and therefore are targeted toward high end users who require the highest possible print quality. The banding phenomenon in this mode is an undesirable print artifact and is considered unacceptable to most users. FIG. 3 shows a printed image produced by an inkjet printer, effectively 60.times. magnified, in which the banding phenomenon is pronounced (see, for example those portions of the image identified by numeric designators 31-33).
The cause of the banding phenomenon is thought to be a combination of many contributing factors that result in an inaccuracy in the positioning of one particular nozzle of the black ("K") pen. For example, print head related errors, such as differences in nozzle directionalities; and paper related errors, such as those introduced by printing from a planar print head nozzle array to a curved recording medium, and by sine/cosine deviations of the printer paper gear mechanism from a perfect circle are all thought to contribute to the banding problem.
In some instances, the banding problem may be attributed to particular print head nozzles. For example, among all the nozzles operating in a black print head having 96 nozzles, the 48th nozzle may be positioned a little too low, i.e. too close to the 49th nozzle. As shown in FIG. 3, under a video microscope with 60.times. magnification, a narrow faint white line followed by a dark band (e.g. bands 31-33) is clearly visible. As can be seen in the figure, not enough ink is deposited by the print head above the 48th nozzle line, while an overlap between the 48th and 49th nozzles shows that too much ink is deposited by the print head below the 48th nozzle line.
The banding artifact occurs in the color enhanced print mode, which is a 2-pass printing mode. FIG. 4 is a schematic diagram that shows the recording medium advanced one-half of the print head size after the first pass printing is completed, and before the second pass printing starts. It has been determined that the banding frequency is substantially one-half the print head width, i.e. every 48 nozzle lines in a 96-nozzle print head.
In the 2-pass print mode, each paper location is swept by the print head exactly twice. In the first pass (40), the print head deposits ink onto the recording medium for 96 pixel lines with one-half of the nozzles muffled by a mask. Then the print head advances for one-half of its size, that is, 48 pixel lines. In a second pass (42), the print head again deposits ink onto the recording medium with the other half of the nozzles muffled by another mask that must be complementary to the first mask. As shown in FIG. 4, there is a mask for the upper half of the first pass, referred to as 1-U; and a mask for the lower half of the first pass, referred to as 1-L. Similarly, masks are provided for the second pass, referred to as 2-U and 2-L, respectively. It can be seen in the figure that the first pass mask 1-U complements the second pass mask 2-L; and that the first pass mask 1-L complements the second pass mask 2-U.
The print masks used in most of the state of the art printers are variations of a checkerboard pattern. See, for example L. Lin, S. Classen, C. -H. Tsao, Spot Deposition For Liquid Ink Printing, U.S. Pat. No. 4,748,453, May 31, 1988. FIG. 5 is a schematic diagram that shows a 1 by 1 checkerboard print mask. Notice that the lower half of the print mask (i.e. nozzles 41-48) is a mirror image of the upper half of the print mask (i.e. nozzles 49-56). The disruption in checkerboard pattern along 48th and 49th nozzle lines is not a contributing factor to the banding problem. For further discussion of various known masking techniques, see M. Hickman, Print Quality of Dot Printer, U.S. Pat. No. 4,965,593, Oct. 23, 1993 (a technique of using multiple nozzles per pixel or pixel row to mask nozzle defects, including shingling and the use of multiple nozzles to fill a given row of data); M. Hickman, Printing of Pixel Locations By An Inkjet Printer Using Multiple Nozzles For Each Pixel or Pixel Row, U.S. Pat. No. 4,963,882, Oct. 16, 1990 (using multiple nozzles per pixel or pixel row to mask nozzle defects; e.g. shingling techniques where multiple nozzles are used to fill a given row of data). It should be noted that all known print masks rely on masking nozzle defects exclusively by patterning the print mask, but do not permanently muffle defective nozzles.
Another problem encountered with inkjet printers is that of ink migration, in which droplets of ink coalesce during printing. Thus, if two neighboring pixels are to be printed, two droplets of ink are deposited onto the recording medium surface as the print head passes. Either the volume of ink is sufficiently large that the droplets of ink flow or bleed into each other before they can dry, or the surface of the recording medium lacks sufficient porosity to absorb the ink droplet of one pixel before an adjoining ink droplet is deposited onto the recording medium surface. The result in either case is run-together or improperly formed pixels.
The problem of ink migration is exacerbated when there are multiple pixels formed adjacent one another before sufficient time has elapsed to allow the ink to dry, such that the recording medium becomes saturated with a pool of ink. See, M. Hickman, Print Quality of Dot Printer, U.S. Pat. No. 4,965,593, Oct. 23, 1993, discussed briefly above.
While known techniques improve print quality, the banding and ink migration problems persist. It would be advantageous to provide a simple and effective solution that reduces or substantially eliminates banding and ink migration artifacts, while requiring only minimal modifications to current printer implementations.